1. Field of the Invention
This invention relates to an apparatus for controlling a hydraulic elevator and, more particularly, to a kind of hydraulic elevator vibration damping control such that the flow rate of pressure oil controlled by variable-speed-driving a rotating machine directly coupled to a hydraulic pump.
2. Description of the Related Art
In conventional hydraulic elevators, the speed of the elevator car is controlled by rotating an electric motor at a constant speed and adjusting with a flow rate control valve the rate at which constant-discharge oil is returned from a hydraulic pump to a tank when the elevator car is lifted up and by controlling falling of the elevator car caused by the weight thereof with the flow rate control valve when the elevator car is lowered. This method entails large energy losses and a large increase in the oil temperature because a surplus amount of oil is circulated during lifting and because the potential energy is consumed by heat development in oil during lowering. Recently, a method, such as the one disclosed in Japanese Patent Publication No.64-311, has been proposed in which an induction motor is controlled by variable-voltage variable-frequency control (hereinafter referred to as VVVF control) to control the discharge from a pump directly coupled to the induction motor in a variable control manner. In this method, only a necessary amount of oil is supplied during lifting and the electric motor is operated for regenerative braking, so that energy losses are reduced and the increase in the oil temperature is very small, thereby realizing a high-efficiency hydraulic elevator system.
FIG. 3 is a diagram of the construction of a controller for a hydraulic elevator using a combination of a plunger and a rope and based on the hydraulic elevator operation principle disclosed in Japanese Patent Publication No.64-311.
Referring to FIG. 3, a cylinder 1 is embedded in a pit of an elevator shaft, pressure oil 2 is charged in the cylinder 1, and a plunger 3 is supported by the pressure oil. A deflector sheave 4 is attached to the top end of the plunger 3. A rope 5 is fixed at its one end to the pit and is wrapped round the deflector sheave 4. An elevator car 6 is connected to the other end of the rope 5. A rail 7 serves to guide the car 6. An electromagnetic changeover valve 8 ordinarily functions as a check valve but can be changed to allow a communication in the reverse direction by energization of an electromagnetic coil. A pipe 8a is connected between the cylinder 1 and the electromagnetic changeover valve 8 to supply pressure oil. A hydraulic pump 9 is operated in a reversible manner to supply pressure oil to the electromagnetic changeover valve 8 or receive pressure oil from this valve through a pipe 9a. An oil tank 10 in which oil is reservoired is provided and oil is supplied from the oil tank 10 to the hydraulic pump 9 or returned from the hydraulic pump 9 to the oil tank 10 through a pipe 10a. A three-phase induction motor 11 drives the hydraulic pump 9 by applying a torque T to the hydraulic pump 9. A velocity generator 12 serves to detect revolutions of the three-phase induction motor 11 and outputs a voltage proportional to the number of revolutions N of the three-phase induction motor 11. A converter 14 converts three-phase AC currents from a three-phase AC power supply 13 into a DC current. A converter 15 supplies regenerated power to the three-phase power source. An inverter 16 receives the DC current from the converter 14 and pulse-width-control this current to generate variable-voltage variable-frequency three-phase currents. A speed controller 18 receives a car 6 speed command 17a, a pressure balance command 17b and the number of revolutions N of the three-phase induction motor 11 to output a control signal 18a to the inverter 16. The pressure balance command 17b is issued prior to the car speed command at the time of starting a movement of the car 6 to rotate the three-phase induction motor 11 at a low speed such that the pressures in the pipes 9a and 8a are equalized while the electromagnetic changeover valve 8 is closed. Variable-voltage variable-frequency control is effected between the three-phase induction motor 11 and the inverter 16 although it is not illustrated, and the three-phase induction motor 11 can output to the hydraulic pump 9 torque T proportional to the control signal 18a to the inverter 16.
FIGS. 4 and 5 show examples of patterns of car speed command 17a, pressure balance command 17b given to the speed controller 18 during lifting and lowering, respectively. The operation of the hydraulic elevator controller shown in FIG. 3 will be described below with respect to the commands shown in FIGS. 4 and 5.
The lifting operation will be described below first with reference to FIG. 4. While the electromagnetic changeover valve 8 is closed and while the three-phase induction motor 11 is stopped, pressure balance command 17b such as that shown in FIG. 4 is supplied to the speed controller 18 at a time t.sub.0. The speed controller 18 thereby outputs control signal 18a. Since as mentioned above the inverter 16 and the three-phase induction motor 11 are VVVF-controlled, the three-phase induction motor 11 outputs torque T in accordance with control signal 18a to the hydraulic pump 9, and the three-phase induction motor 11 and the hydraulic pump 9 start rotating to produce a pressure in the pipe 9a. At this time, a load torque is produced in the hydraulic pump 9 in accordance with the pressure in the pipe 9a. However, the number of revolutions N of the three-phase induction motor 11 is returned to the speed controller 18 and the number of revolutions N of the three-phase induction electric motor 11 is increased in accordance with pressure balance command 17b, as shown in FIG. 4.
The pressure in the pipe 9a connected to the electromagnetic changeover valve 8 becomes equal to the pressure in the pipe 8a at a time t.sub.1. Then the electromagnetic changeover valve 8 is opened. At a time t.sub.2, car speed command 17a is issued as illustrated. During lifting operation, the induction motor 11 is revolution command is expressed as the sum of car speed command 17a and pressure balance command 17b. The three-phase induction motor 11 and the hydraulic pump 9 therefore rotate at a high speed, and oil in the oil tank 10 flows into the cylinder 1 through the pipes 10a, 9a, and 8a to move the plunger 3 and the deflector sheave 4 upward. Since the rope 5 is wrapped round the deflector sheave 4, the deflector sheave 4 is rotated to move the car 6 to an extent twice as large as the extent to which the plunger 3 is moved. Car speed command 17a is successively changed to move the position of the car 6. When the car 6 moves to the desired position, the electromagnetic changeover valve 8 is closed to stop the car 6.
Next, the car lowering operation will be described below with reference to FIG. 5. The operation is the same as the lifting operation with respect to the initial step from rotating the three-phase induction motor 11 in accordance with pressure balance command 17b to opening the electromagnet valve 8. However, the polarity of car speed command 17a is opposite to that of pressure balance command 17b as shown in FIG. 5, so that the number of revolutions of the three-phase induction motor 11 is reduced and the three-phase induction motor 11 starts rotating in the lowering direction at a time t.sub.3. Pressure oil 2 in the cylinder 1 is thereby recovered to the oil tank 10 through the pipes 8a, 9a, and 10a, and the car 6 is lowered. At this time, the hydraulic pump 9 receives a load in a direction opposite to the direction of its rotation, and the converter 15 regenerates power to the three-phase power supply 13.
A block diagram such as that shown in FIG. 6 is obtained by adding a speed feedback of the three-phase induction motor 11 to a basic formula expressing a vibrating motion during the operation of the hydraulic elevator shown in FIG. 3, that is, when the electromagnetic changeover valve 8 is open.
A block 19 shown in FIG. 6 within a dotted rectangle corresponding to the speed controller 18 designates a coefficient which represents the relationship between the car speed and pump revolutions. A.sub.J is a sectional area of the plunger 3, and V.sub.0 is a theoretical displacement of the hydraulic pump 9 per radian revolution. A block 20 designates a transfer function with respect to a signal representing the difference between the rotating speed of the induction motor 11 designated by the revolution command and the actual rotating speed. Control signal 18a is formed by this function. By a power supply system constituted by components 11, 13, 14, 15, and 16, torque T is output from the induction motor 11. A block 21 designates a function constituted by a moment of inertia Jeg of the induction motor 11 and the hydraulic pump 9 and a Laplacean S. Torque T is converted into the rotating speed of the induction motor 11, i.e., the number of revolutions N through this function. A block 22 designates a coefficient for conversion of the speed of the induction motor 11 into the speed of the car 6, which is, of course, reciprocal of coefficient 19. A block 23 designates a coefficient representing a vibration system determined by the elasticity of pressure oil in the cylinder 1, the mass of the plunger 3, the mass of the car 6 and the elasticity of the rope 5, and .tau..sub.0 is a time constant of this vibration system. By conversion of this coefficient, a car speed Xc is obtained. A block 24 designates a function for converting the car speed Xc into a pressure P.sub.1 of pressure oil 2 in the cylinder 1, the pipes 8a and 9a and the hydraulic pump 9. The load imposed upon the hydraulic pump 9 is obtained by multiplying pressure P.sub.1 by a theoretical displacement 25 of the hydraulic pump 9 per radian revolution. The gain of transfer function 20 is set to a high level in order to rotate the induction motor 11 in response to pressure balance command 17b and car speed command 17a by prevailing over the load imposed upon the hydraulic pump 9. The variation in the speed of the induction motor 11 in the case of vibration at car speed Xc and time constant .tau..sub.0 is therefore very small. That is, no vibration component appears in the result of detection of the rotational speed of the induction motor 11.
However, the coefficient 23 representing a vibration characteristic of the hydraulic mechanical system as shown in FIG. 6 contains no attenuation term. The control system therefore entails a drawback such that if vibration corresponding to a pole of the hydraulic mechanical system (natural frequency: 1/.tau..sub.0) is caused by a change in speed pattern during traveling operation or a certain shock, it lasts for a long time, so that the passenger has a feeling of uncomfortableness.